In recent years, materials which are non-linear between a polarization and an electric field, said fields appearing after exposure to a strong optical field such as a laser light, have attracted attention.
Such materials are generally known as non-linear optical materials, and are described in detail in the following publications: "Nonlinear Optical Properties of Organic and Polymeric Material", ACS SYMPOSIUM SERIES 233, edited by David J. Williams (American Chemical Society, 1983); "Organic Nonlinear Optical Materials", under the supervision of Masao Kato and Hachiro Nakanishi (C.M.C. Company, 1985); and "Nonlinear Optical Properties of Organic Molecules and Crystals", Vols. 1 and 2, edited by D. S. Chemla and J. Zyss, (Academic Press Company, 1987).
One utility of the non-linear optical material is as a wavelength conversion device, useful in the development of a second harmonic generation (SHG) which is based on a secondary non-linear effect and an addition of frequencies and a subtraction of frequencies. What has been used actually heretofore is an inorganic perovskite typified by lithium niobate. In recent years, it has been found that a .pi.-electron conjugated-type organic compound, having an electron donative group and an electron attractive group, has various properties as a non-linear optical material, which greatly upgrades the above-mentioned inorganic material.
For the formation of a secondary non-linear optical material having higher properties, compounds having high non-linear sensitivity in a molecular state should be aligned so as not to generate inverse symmetry. It is known that for the generation of high non-linear sensitivity, compounds having a long .pi.-electron conjugated chain are useful. As described in the above-mentioned publications, it is evident that in these .pi.-electron conjugated compounds, the absorption maximum wavelength increases, and the transmittance of blue light decreases. This result is detrimental with respect to generation of blue light as a second harmonic. This also occurs when using a p-nitroaniline derivative. For generating a second harmonic, the effect of the transmittance of the p-nitroaniline derivative's wavelength is great as is shown in Plain Azema, et al., Proceedings of SPIE, Vol. 400, New Optical Materials, (1983), page 186, FIG. 4.
Accordingly, a non-linear optical material having a high transmittance to a blue light is desired. Heretofore, investigations have been made to substitute a carbon atom with a nitrogen atom in a benzene ring of nitroaniline, but no satisfactory result has been obtained.
Further improvement has been attempted and many materials are shown in JP-A-62-59934, JP-A-63-23136, JP-A-63-26638, JP-B-63-31768. JP-A-63-163827, JP-A-63-146025, JP-A-63-85526, JP-A-63-239427, JP-A-1-100521, JP-A-64-56425, JP-A-1-102529, JP-A-1-102530, JP-A-1-237625 and JP-A-1-207724. (The term "JP-A" means unexamined published Japanese patent application and "JP-B" means examined Japanese patent publication.)
The present inventors have disclosed an excellent method in JP-A-62-210430 and JP-A-62-210432.
Particularly, 3,5-dimethyl-1-(4-nitrophenyl) pyrazole, disclosed in JP-A-62-210432, provides for a non-linear optical constant of maximum value, which comprises a tensor component of an off diagonal element capable of phase matching among heretofore known organic non-linear optical materials, and is a very useful compound.
In addition to the transmission of the second harmonic to be generated as mentioned above, attention should be paid to the transmission of the fundamental wave to be contained in the non-linear optical material. This has been widely studied in the investigations of plastic optical fibers. It has been made clear that in the case of organic high molecular materials, an absorption is seen in the vicinity of 1 .mu.m, and results in propagation loss. It is considered that this absorption in the vicinity of 1 .mu.m is due to the triplicate wave of stretching vibration of a carbon-hydrogen bond, a nitrogen-hydrogen bond, or an oxygen-hydrogen bond. By replacing the hydrogen atom of these bonds with a deuterium atom, the absorption strength can be curtailed, as described in JP A-54-65556, JP-A-57-81204, JP-A-57-142601, JP-A-58-149003, JP-A-58-154803, JP-A-61-20906, and JP-A-61-223805.
In the field of organic non-linear optical materials, a description of deuterium compounds is seen in JP-A-49-81051, JP-A-63-21627, JP-A-63-163825, JP-A-2-149825, and JP-A-2-247620. DLAP (Deutrated L-Arginine Phosphate) obtained by partly deuteration of LAP is well-known. However, these described compounds do not have sufficient properties, and deuterated compounds derived from compounds having excellent properties are desired.